1. Field of the Invention
The present invention relates to an ink jet head, and an ink jet recording apparatus which uses such head.
2. Related Background Art
In the conventionally known ink jet head, means for generating energy is arranged on the liquid paths for discharging ink, while the ink supplied to a liquid chamber is induced from an ink tank or the like to the above-mentioned liquid paths through an ink supply outlet. Here, the energy is generated and given to the ink by means for generating energy, thus discharging the ink from the discharge ports. The discharged ink droplets impact upon a recording medium to form pixels for recording. Of such ink jet heads, the one which utilizes thermal energy for discharging a recording liquid (ink) is capable of forming a plurality of discharging ports in a high density. Therefore, in addition to the capability to record in a high resolution, a head of the kind has an advantage that the head can be easily fabricated compactly as a whole.
In the conventional ink jet head which utilizes the thermal energy, a structure is generally arranged so that such a high density is attained by arranging a plurality of heat generating resistive elements on a substrate made by such as silicon, and also, using a base board having a common layer of heat accumulation and an electrically insulated layers.
FIG. 11 is a view schematically showing the cross-section of an ink jet head of such a structure represented as a background art.
In FIG. 11, a reference numeral 1 designates an alumina substrate; 2a, a heat accumulation layer; 2b and 2c, electrically insulated layers, respectively; 3, the heat generating resistive elements; 3a, electrode wiring to supply electric power to the heat generating resistive elements 3; and 4, a protective film against cavitation.
On a heater board (hereinafter may be referred to as laminated circuit base board) 100 structured as above, a ceiling board 400 grooved to form liquid paths 401 and others is joined together to constitute an ink jet head.
As a result, when the heat generating elements are energized to execute discharges, the heat thus generated is not necessarily used for discharging only, but a part of it is released to the liquid paths adjacent thereto through the aforesaid base board and others.
With a head of such a structure, the volume of discharged ink droplet becomes comparatively large because the generated heats themselves give influence to each other when ink is discharged from all the discharge ports at a time, that is, the aforesaid plural heat generating resistive elements are energized at a time to generate heat. On the other hand, when a heat generative resistive element is energized to generate heat individually, the discharging volume becomes comparatively small as compared with the above-mentioned situation. As a result, in a case where a heat generating resistive element is driven individually, it is necessary to make an arrangement so that the element can generate thermal energy in an intensity greater than usual if the same discharging volume should be obtained as in the full discharge. In other words, the difference occurs in the foaming efficiency depending on the energy to be given, necessitating an adjustment of the discharging volumes at the time of an individual discharge and a full discharge. Thus the configuration of electric power-supply becomes inevitably complicated. If the volume of the full discharge is made a standard without any adjustment, the individual discharging volume tends to be insufficient, leading to a significantly inferior quality of a recorded image.
FIG. 12 is a line diagram showing the relationship between the voltage P applied to the head and the discharging volume Vd.
As clear from FIG. 12, there is a great difference in the discharging volumes of individual and full discharges particularly in the electric power P'.sub.TH when the discharge begins. This difference takes place in spite of the fact that the electric power is equally required for both of them to obtain the temperature good enough to create film boiling in ink. This is due to the difference in the heat accumulation in the foaming portions after the heat generating resistive elements generate heat. When a discharge is made only from an individual nozzle, the heat radiation is greater to the adjacent liquid paths and others, and before the size of the maximum foaming becomes large enough, the defoaming occurs, while in a case where a plurality of heat generating resistive elements are driven at a time such as a full discharge, the heat radiation to the adjacent liquid paths and others is not so great, and the size of foaming becomes large enough accordingly.
In other words, it is necessary to apply an electric power equivalent to the P'.sub.OP which is greater than the P'.sub.TH as shown in FIG. 12 in order to obtain the same discharging volume for the individual discharge and the full discharge in the conventional head.